1. Field of the Invention
This invention relates to a semiconductor die-attach technique and composition, and more particularly to a technique employing a composition suitable for remelting during subsequent assembly operations.
2. Description of the Prior Art
Semiconductor die are typically attached to a package by means of a bonding composition formed into plaques approximately the size of the die for ease of handling, which composition is typically made from various metals or polymers. A few examples of high-temperature melting die-attach compositions are gold-tin melting at approximately 280.degree. C.; gold-silicon melting at approximately 363.degree. C.; or gold-germanium melting at approximately 356.degree. C. Such die-attach compositions have been adequate in the past for attaching relatively small die to packages. However, when the die is much larger in size, for example above 0.250 in.sup.2 (VLSI size), such die-attach compositions have proven to be inadequate.
During a typical assembly operation, the die is placed in a cavity of a package containing the die-attach composition. Heat is applied to the package containing the die so as to melt the die-attach composition, which bonds the die to the package. Subsequently, a lid is placed over the die-receiving cavity of the package and heat is again applied so as to seal the lid to the package and form an hermetic enclosure for the integrated circuit. Such lid-sealing temperatures are typically 400.degree. C. to 450.degree. C.
When the prior art gold-tin die-attach composition is heated to such elevated temperatures, it remelts and dissolves gold and silicon into the melt. Gold-silicon and gold-germanium also remelt. They do not dissolve additional gold and silicon as readily, but instead form "brittle" or "hard" solder bonds which tend to crack VLSI size semiconductor die during cooling and temperature cycling.
Several types of die bond materials containing powders are presently available commercially. Epoxies are the most well known and have the longest history of use, (at least 10 years). They basically consist of an inorganic filler phase, either metal or ceramic, and an epoxy resin to bind the particles and to provide adhesion to the chip and substrate or package. Metallic fillers such as silver, gold and silver alloys are routinely combined with epoxy resins to produce electrically conductive composites for die bond applications. Insulating fillers such as magnesium oxide (MgO), aluminum oxide (Al.sub.2 O.sub.3), beryllium oxide (BeO), and boron nitride (BN) are often employed where electrical insulation between the die and substrate is required.
Organic resins range from diglicydilether of bisphenylA (low glass transition temperature) to cycloaliphatic epoxides (high glass transition temperature). Most come premixed and must be frozen prior to use to retard curing. Some, however, are two part systems. These consist of separate resin and hardener phases. Each is usually premixed with the desired filler phase. Just prior to use, the resin and hardener are mixed together and used for die bonding. Some of these systems cure at room temperature, others require temperatures as high as 180.degree. C.
Metal filled epoxy systems normally have electrical resistivity values five to ten times higher than the pure metals and exhibit correspondingly poor thermal conductivity. Above 200.degree. C. the epoxies degrade and if the temperature is increased further they give off volatile organic products.
Another class of organic die bonding materials is polyimides. These materials employ virtually the same metal systems as epoxies. Polyimides, however, possess much greater temperature stability. They cure between 250.degree. C. and 350.degree. C. and can withstand limited excursions to 400.degree. C. They possess the same electrical and thermal conductivity limitations as epoxies. Additionally, both of these organic systems contain impurities such as sodium chloride which can enhance the chance of chip corrosion.